The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting an end-point of a metal layer during a chemical mechanical polishing operation.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. For example, the substrate may be transferred to a metrology station where the thickness of a substrate layer is measured, e.g., with a profilometer or a resistivity measurement. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This is a time consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable.
Several methods have been developed for in-situ polishing endpoint detection. Most of these methods involve monitoring a parameter associated with the substrate surface, and indicating an endpoint when the parameter abruptly changes. For example, where an insulative or dielectric layer is being polished to expose an underlying metal layer, the coefficient of friction and the reflectivity of the substrate will change abruptly when the metal layer is exposed.
Where the monitored parameter changes abruptly at the polishing endpoint, such endpoint detection methods are acceptable. However, as the substrate is being polished, the polishing pad condition and the slurry composition at the pad-substrate interface may change. Such changes may mask the exposure of an underlying layer, or they may imitate an endpoint condition. Additionally, such endpoint detection methods will not work if only planarization is being performed, if the underlying layer is to be over-polished, or if the underlying layer and the overlying layer have similar physical properties.
A method is disclosed for determining an endpoint associated with chemical mechanical polishing a metal layer on a substrate, the endpoint having a predetermined pattern reflected of reflected high intensity. In one aspect, the method includes bringing a surface of the substrate into contact with a polishing pad that has a window; causing relative motion between the substrate and the polishing pad; directing a light beam through the window, the motion of the polishing pad relative to the substrate causing the light beam to move in a path across the substrate; detecting light beam reflections from the metal layer; generating reflection data associated with the light beam reflections; dividing the reflection data into a plurality of radial ranges; and identifying the predetermined pattern from the reflection data in the plurality of radial ranges to establish the endpoint.
Implementations of the invention includes one or more of the following. The chemical mechanical polishing operation is stopped when the endpoint is identified. The reflection data may be stored on a media for subsequent analysis. Processing of the reflection data may be done in real-time or off-line. The identifying step may include comparing the reflection data to a predetermined threshold. The identifying step may include determining whether the reflection data is in a downward trend, an upward trend, or a flat trend. The detecting step measures reflections corresponding to a sampling zone in the path across the substrate and may include determining a radial position for each sampling zone; determining a position of the carrier head from a carrier head sweep profile; and dividing the reflection measurements into a plurality of radial ranges according to the radial position.
In another aspect, a method determines an endpoint associated with chemical mechanical polishing a metal layer, the endpoint having a predetermined pattern of reflected light intensity. The method includes directing a light beam through a window of a polishing pad and moving the polishing pad relative to the substrate to cause the light beam to move in a path across the substrate; detecting light beam reflections from the metal layer; generating reflection data associated with the light beam reflections; dividing the reflection data into a plurality of radial ranges; and identifying the predetermined pattern from the reflection data in the plurality of radial ranges to establish the endpoint.
Implementations of the invention include one or more of the following. The chemical mechanical polishing may be stopped when the endpoint is identified. The reflection data may be stored on a media for subsequent analysis. The identifying step includes comparing the reflection data to a predetermined threshold. The identifying step may also include determining whether the reflection data has a downward trend, an upward trend or a flat trend.
In another aspect, an apparatus for polishing a metal layer of a substrate includes a carrier head to hold the substrate; a polishing pad adapted to contact a surface of the substrate, the polishing pad having a window therethrough; a motor coupled to the polishing pad for causing relative motion between the substrate and the polishing pad; a light source to direct a light beam through the window, the motion of the polishing pad relative to the substrate causing the light beam to move in a path across the substrate; a sensor optically coupled to the light source for detecting light beam reflections from the substrate, the sensor generating reflection data associated with the light beam reflections; an electronic bin coupled to the sensor for separating the reflection data into a plurality of radial ranges; and a pattern recognizer coupled to the sensor and to the bin for identifying the endpoint by comparing the predetermined pattern to the reflection data.
Implementations of the invention include one or more of the following. A polishing controller may be connected to the pattern recognizer, the polishing controller stopping chemical mechanical polishing when the endpoint is identified. The reflection data may be stored on a media for subsequent analysis. The pattern recognizer compares the reflection data to a predetermined threshold. The pattern recognizer may determine whether the reflection data has a downward trend, an upward trend or a flat trend.
Advantages of the invention include one or more of the following. The reflection data from a wafer is captured using a high resolution data acquisition system at a relatively fine time scale, on the order of milliseconds. Further, reflection intensity changes during polishing are captured for different radial positions on the substrate. The high resolution data acquisition system provides precise time control of each process step in a multi-step operation. Detailed data is available on the progress of the metal polishing operation at different locations of the wafer. Additionally, parameters such as uniformity of the entire wafer and removal rate for different radial portions of the wafer are determined. The acquired high resolution data can be processed on-line or off-line to adjust various variables and parameters to minimize erosion and dishing of the surface layer. If the data is processed in real-time, the feedback data may be used for endpoint detection or for closed-loop control of process parameters. For instance, the polishing pressure, polishing speed, chemistry, and slurry composition may be altered in response to the feedback data to optimize the overall polishing performance and/or polishing quality. The reflection data is available for experimentation to improve the deposition process.
Other features and advantages of the invention will become apparent from the following description, including the drawings and claims.